When a polyester is treated at elevated temperatures with a diol, the expected reaction is a transesterification reaction in which diol moieties of the polyester chain are liberated and new diol moieties are inserted into the polyester chain. In contrast, the present inventors have unexpectedly discovered that where the diol reactant is different from that of the diol moieties in the starting polyesters, copolymers are obtained. More specifically, the present inventors have determined that dihydroxy aromatic compounds inserted into the chain of a polyester which is prepared from a dicarboxylic acid and an aliphatic diol, have predominantly ether linkage rather than ester linkages. Thus, poly(ethylene terephthalate) treated with hydroquinone, for example, has mono- and diether linkages. Incorporation of the dihydroxy aromatic compounds into the polyester chain causes an increase in the glass transition temperature (Tg) value and the thermal stability of the polyester. For example, the Tg value of poly(ethylene terephthalate) has been increased from 72.degree. C. to 100.degree. C. by modification with certain dihydroxy aromatic compounds.